Motor vehicles, as presently manufactured, are equipped with systems for defogging and deicing windshields. Generally, the systems depend upon heat generated in the internal combustion engine and transferred to the engine's cooling system to be blown as warm air across the interior of the windshield to accomplish the defogging and deicing. In such a case, of course, it is readily apparent that there is a period of time between the starting of an engine and the time that sufficient heat is being generated in its cooling system in order to provide a defogging and/or deicing of the vehicle's windshield. Depending upon the exact temperature conditions and the time the vehicle has been sitting idle without its engine running, the period of time before sufficient heat is available to accomplish this function can be up to ten minutes or more.
In view of the fact that there can be a rather lengthy delay before the present day motor vehicle's heating and defrosting system can clear a windshield, automotive designers have been attempting to design systems which generate heat from electrical energy to accomplish a relatively rapid defrost and deicing of a vehicle windshield. Such an electrically heated defrosting and deicing system generally would be independent of the normal heating and defrosting system contained in the motor vehicle.
Many different systems have been purposed for accomplishing this rapid defrost and deicing function, including the placement of an electrically conductive transparent coating on the windshield and embedding fine wires in a laminating inter layer of the windshield.
Another feature that is desirable in such an electrically heated windshield is a uniform defogging and deicing of the windshield without any hot spots. By this, I mean that a gradual and uniform defogging or deicing takes place on the windshield.
No search was conducted on the subject matter of this specification in the United States Patent Office or in any other search facility. I am, however, aware of my previously issued U.S. Pat. No. 4,543,466 for a Bus Bar Arrangement for Uniformly Heating a Trapezoidally-Shaped Electrically Heated Windshield. This patent issued on Sept. 24, 1985 and is also assigned to Ford Motor Company who is the assignee of this application.
My aforementioned patent discloses an electrically heated windshield of generally trapezoidal shape. This windshield has bus bars of uniform conductivity throughout their length and extending in line contact with the upper and lower edges of a continuous uniformly thick trapezoidally-shaped electrically conductive coating provided on the windshield surface for deicing and defrosting the windshield. The line of contact of the upper bus bar with the conductive coating has a length generally equal to the entire effective length of the upper edge of the conductive coating. The lower bus bar is symmetrically located along the lower edge of the conductive coating and has a line of contact length equal to the sum of the length of the upper bus bar plus generally about one-half the difference between the entire effective length of the lower edge of the conductive coating minus the length of the line of contact of the upper bus bar.
In my aforementioned patent, I noted that I had personally conducted a search in the U.S. Patent Office with respect to the subject of that patent. I reported as a result of my search that the U.S. Pat. Nos. 957,728; 3,313,920; 3,947,618 and 4,361,751 were of interest with respect to the subject matter of my previous application. However, these four patents are not of interest with respeot to the subject matter of this application and therefore no further discussion thereof will be undertaken. The reader is referred to my aforementioned patent for a brief summary of each of these four patents.